The present invention generally relates to systems for generating plasma between two electrodes of a spark plug, these systems being particularly used for the controlled radio-frequency ignition of a gas mixture in combustion chambers of an internal combustion engine.
For such an application to automobile ignition with plasma generation, plasma-generating circuits incorporating coil-spark plug assemblies are used to generate multifilament discharges between their electrodes, making it possible to initiate combustion of the mixture in the combustion chambers of the engine. The multi-spark plug is described in detail in the following patent applications filed in the name of the applicant: FR 03-10766, FR 03-10767 and FR 03-10768.
Such a coil-spark plug assembly is conventionally modeled by a resonator 1 whose resonant frequency Fc is greater than 1 MHz, typically close to 5 MHz. The resonator comprises in series a resistor R, an inductor L and a capacitor C. Ignition electrodes 10 and 12 of the coil-spark plug assembly are connected to the terminals of the capacitor C.
When the resonator is supplied with a high voltage at its resonant frequency fc (1/(2π√{right arrow over (L*C))}, the amplitude at the terminals of the capacitor C is amplified, making it possible to develop multifilament discharges between the electrodes of the spark plug over distances of the order of a centimeter, with a high pressure and for peak voltages of less than 20 kV.
The sparks are then referred to as branched sparks in so far as they entail the simultaneous generation of at least a number of ionization lines or paths within a given volume, their branches additionally being omnidirectional.
Controlling the power supply of such a coil-spark plug assembly requires the use of a supply circuit that is capable of generating voltage pulses, typically of the order of 100 ns, which may reach amplitudes of the order of 1 kV, at a frequency intended to be very close to the resonant frequency of the radio-frequency resonator of the coil-spark plug assembly. The smaller the difference between the resonant frequency of the resonator and the operating frequency of the generator, the higher the overvoltage coefficient of the resonator (ratio between the amplitude of its output voltage and its input voltage).
Such a supply circuit, described moreover in detail in patent application FR 03-10767, is schematically represented in FIG. 2. It conventionally employs a “Class-E power amplifier” setup. This type of DC/AC converter makes it possible to create the voltage pulses with the aforementioned characteristics.
According to the embodiment in FIG. 2, the amplifier 2 comprises a MOSFET power transistor M used as a switch for controlling the switchings at the terminals of the resonator 1.
Thus, a control device 5 generates and applies a control signal V1 at a control frequency to the gate of the power MOSFET M, via a control stage 3 which is represented schematically. In order to control the production of sparks between the electrodes of the coil-spark plug assembly connected at the output of the amplifier when its resonator 1 is excited by means of the control signal V1, said signal is not permanent but is present in the form of control pulse trains at the control frequency.
As described in patent application EP-A-1 515 594, a parallel resonant circuit 4 is connected between a source of intermediate voltage Vinter and the drain of the transistor M. This circuit 4 comprises an inductor Lp in parallel with a capacitor Cp.
Close to its resonant frequency, the parallel resonator converts the intermediate voltage Vinter into an amplified voltage Va (illustrated in FIG. 5), corresponding to the intermediate voltage multiplied by the overvoltage coefficient of the parallel resonator. This amplified voltage is provided on the drain of the transistor M connected moreover to the input of the resonator 1.
The transistor M therefore acts as a switch and applies (or blocks) the voltage Va at the input of the resonator 1 when the control signal V1 is in the high (or low) logic state. The transistor M thus imposes a switching frequency, determined by the control signal V1, which is sought to be made as close as possible to the resonant frequency of the coil-spark plug assembly connected at the output (typically 5 MHz), in order to maintain and to maximize the transfer of energy between the parallel resonator 4 and the series resonator 1 forming the coil-spark plug assembly.
At the resonant frequency of the coil-spark plug assembly, the output voltage Va mentioned above, multiplied by the overvoltage coefficient of the series resonator 1, then appears at the terminals of the capacitor C of the series resonator 1, that is to say at the terminals of the electrodes at the spark plug.
This phase of energy transfer from the power stage formed by the amplifier to the resonator of the coil-spark plug assembly must be carried out at the resonator frequency of the resonator in order to ensure good efficiency. Specifically, if the transistor M imposes a switching frequency which differs from the resonant frequency of the coil-spark plug assembly, the energy transfer is degraded owing to the narrowness of the passband of the series resonator used for the coil-spark plug assembly.
In an application to automobile ignition with plasma generation, each combustion chamber is equipped with a coil-spark plug assembly as described above in order to initiate combustion on command.
Consequently, in the case of 4-cylinder engines, for example, four supply circuits of the class-E amplifier type, as described above with reference to FIG. 2, must be able to be made available in order to supply and control the four coil-spark plug assemblies respectively.
Such a configuration thus based on having as many amplification paths as there are coil-spark plug assemblies to be controlled then limits the development potential of this type of automobile ignition by plasma generation, not only because of the space requirement entailed by this installation below the engine hood, but also because of the installation cost, which may prove to be prohibitive for envisioning the installation of this type of ignition in mass-produced vehicles.